1. Field of the Invention
The present invention relates to a manufacturing method for a thermal head.
2. Description of the Related Art
There have been conventionally known a thermal head which is used in a thermal printer often equipped in a compact information terminal typified by a compact hand-held terminal, and which is used to perform printing on a thermal recording medium based on printing data with the aid of selective driving of a plurality of heating elements (for example, see JP 2007-83532 A).
As to increasing efficiency of the thermal head, there is known a method of forming a heat insulating layer in a lower layer of a heating portion of a heating resistor. When the heat insulating layer is formed in the lower layer of the heating portion, among an amount of heat generated in the heating resistor, an amount of upper-transferred heat which is transferred to a wear-resistant layer formed above the heating portion becomes larger than an amount of lower-transferred heat which is transferred to an insulating substrate located under the heating portion, and thus energy efficiency required during printing can be sufficiently obtained. In the thermal head described in JP 2007-83532 A, owing to a substrate including a concave portion, a hollow portion is formed below a heating portion of a heating resistor, and the hollow portion is caused to function as a void heat insulating layer. That is, heat transfer in a thickness direction of the substrate is prevented by means of the hollow portion, and accordingly, sufficient heat storage performance is obtained. It should be noted that the substrate for forming the hollow portion is formed by employing a fusion method in which a glass substrate including a concave portion and a flat glass substrate are bonded to each other at a temperature of about 500° C. or higher.
However, glass has a property of shrinking in a heat cycle, and thus a position of the concave portion (void heat insulating layer) formed on the glass substrate varies between before and after the bonding. In addition, a heat shrinkage percentage of glass varies depending on a composition of the glass substrate or conditions (for example, temperature, heating time, and the like) of the heat cycle. For this reason, when a thin-film-like heating resistor is formed in the heating element forming step, a pattern misalignment (position misalignment) occurs between the concave portion and the heating portion of the heating element due to heat shrinkage occurring in the bonding step, leading to inconvenience that the heating portion cannot be accommodated in the concave portion of the substrate. The pattern misalignment as described above reduces a heat insulating effect of the substrate.
Further, when a large number of thermal heads are collectively manufactured on a large substrate, an effect of the pattern misalignment due to the heat shrinkage percentage becomes particularly serious in accordance with a position of the thermal head. This leads to a decrease in yield to obtain a thermal head having high energy efficiency. The effect of the heat shrinkage on the substrate may be reduced, in some cases, by using a photo mask which is manufactured by taking the heat shrinkage percentage into consideration. However, there are variations in heat shrinkage percentage, and hence it is difficult to deal with the pattern misalignment due to the heat shrinkage only by correction using a correction mask.